Core: Reimplemented the behaviour of CSS/JS timestamps

Problem:
On a production website, end users often need to refresh a page or clear local browser cache to see the last changes made to a stylesheet or javascript file.

Cause:
The time-stamping feature is **never used** in non-debug mode.

Solution:
Enable time-stamping feature by default (when parameter is not given and when in non-debug mode).

Notes:
* This also works for non-minified files.
* Don't forget to touch your parameters.yml file when publishing!!!
* @tijsverkoyen once said that the behaviour of adding CSS/JS timestamps would be reimplemented.
  See: https://github.com/forkcms/forkcms/pull/660#issuecomment-38819080.
  I know Tijs is very busy and now that Belgium is out of the World Cup, lets help him out here!